Flood mitigation at the downstream areas of a transboundary river

Floods in the basin of the Ardas river, a transboundary river that has its springs in Bulgaria and its outlet in Greece, have often created havoc and caused millions of damage, especially in downstream Greek areas, which repeatedly receive unregulated flow from upstream dams. More specifically, the Ardas river, a tributary of the Evros river, flows for 241 km in Bulgaria and for only 49 km in Greece and its catchment stretches for 5.200 km2 (94% of the total area) in Bulgaria and for 350 km2 (6% of the total area) in Greece. Three large dams along the river have been constructed in Bulgaria (Kardzhaly, Studen Kladenets and Ivaylovgrad), the last one, Ivaylovgrad dam, being in short distance (approx. 15 km) from the transnational borders. During heavy rain, excessive flow from Ivaylovgrad dam is often released downstream, in order to relieve the reservoir that is kept at maximum level for energy production reasons. As a result, the downstream areas, also affected by the same heavy rain events, need to regulate large flows, often with inadequate response time and relevant means. The present study describes an approach to estimate flood water levels in the Greek territory, caused by both intense rain events and increased releases from the upstream dam. For this purpose the study area was divided into three sub-basins and the corresponding flood volumes were calculated using several methodologies. Given the fact that downstream areas are proved to be in high risk in terms of flooding, a series of structural and non-structural measures for the downstream area is examined and the paper concludes with an approach towards the confrontation and mitigation of flood effects in transboundary river basins.